There is a method for connecting (i.e., bonding) a printed circuit board to a flexible printed circuit board. Specifically, a lead terminal (i.e., a land) of the printed circuit board is soldered to a lead terminal (i.e., a land) of the flexible printed circuit board. The lead terminal of the printed circuit board faces the lead terminal of the flexible printed circuit board. Then, flexible printed circuit board is pressed and heated from the upper side of the flexible printed circuit board by a heating tool (i.e., a heater head) of hot press equipment. Thus, solders on the lead terminals of both the circuit boards melt so that they are bonded together.
After soldering between the lead terminals, a clearance is formed between the printed circuit board and the flexible printed circuit board. Therefore, it is required that the clearance is sealed and filled with resin. The resin also works as reinforcement of a connection between the circuit boards and water protect of the lead terminals.
However, in the above method, it is necessitated to add an additional process for sealing and filling the clearance between the circuit boards with the resin after the terminals are bonded and soldered. Therefore, the number of manufacturing processes in the method is increased, so that a manufacturing cost is increased.
In view of the above problem, another method for connecting a printed circuit board to a flexible printed circuit board is disclosed in Japanese Unexamined Patent Application Publication No. H10-270820. In this method, the flexible printed circuit board is made of thermoplastic resin so that the flexible printed circuit board is melted by heat in case of soldering. Thus, the clearance between the flexible printed circuit board and the printed circuit board is filled with resin composing the flexible printed circuit board.
This method can be performed without sealing the clearance with resin. Specifically, an additional process for sealing and filling the clearance with resin is not necessitated in the method of the prior art. However, when the heater head presses and heats the flexible printed circuit board so that the resin of the flexible printed circuit board is melted, the melted resin is flown out so that the thickness of the flexible printed circuit board is partially thinned. In this case, stress is concentrated at a portion where the thickness becomes thin, so that the circuit boards bonded together have weak mechanical strength.